Third generation radio cellular networks, here refereed to as 3G-networks, are emerging on the market. The 3G-networks provide a multitude of services, especially high bit rate services and multimedia. A general strive when designing a 3G-network is to maximise it's capacity and data, throughput.
The CDMA-access scheme has been proposed to realise these 3G-networks, e.g. W-CDMA in the USA and CDMA-2000 in Europe.
The handover is a process by which the radio access network changes the radio transmitters, radio access modes, or radio systems that are used to provide the bearer services to the UE (User Equipment), while maintaining a defined RAB (Radio Access Bearer).
Handovers from one cell to another are required in several situations. The most common situation is when the UE moves from one base station coverage area to another. The UE may move between stations within the same radio system or into another system. The 3GPP standards support handovers between UTRA-FDD and GSM or time division duplex (TDD) network frequency bands that meet the specifications.
The multi-standard UE may change its frequency or radio access mode, during a handover to a different cell. The UE may need a handover if it's requested service level exceeds the current cell capacity. If a target cell cannot support the combination of bearer services (voice, data, multimedia, etc) that are provided by the current serving cell, some, or all, of the bearer services may be handed over to another cell.
Within the W-CDMA system, handovers are “soft” in order to maintain link reliability and to allow the use of identical carrier frequencies (intra-frequency handovers). In a soft handover, the UE transmits and receives the same signal from both cells simultaneously to make the transition as seamless as possible. Handovers are more complex when a multi-standard UE moves between cells with different carrier frequencies or to a different network, such as GSM (different carrier frequencies). Both types of handover are managed with an assist from the UE mobile unit.
The multi-standard UE continuously monitors for the presence of cells with other frequencies and radio access systems that it supports. When the network senses the need for a handover, Node B measures some system parameters and commands the UE to measure other parameters and report the results. Key parameters include carrier frequency, system type, traffic volume and QoS levels. When a handover is needed, the NODE B directs the UE to operate in a compressed mode. FIG. 1 illustrates an example of compressed mode transmission. The compressed mode is a method of turning off transmissions for a portion of the 10-ms frame to create gaps that allow time for the UE and NODE B to make a prescribed set of measurements, this gap is illustrated by time period C in FIG. 1. Compressed-mode operation can be achieved by decreasing the spreading factor, removing bits from the data (“puncturing”), or using higher level scheduling to allocate fewer timeslots for user traffic. In compressed frames, the transmission gap slots are not used for data transmission and the instantaneous transmit power is increased in those slots to maintain quality (BER, FER, etc.) during the periods of reduced processing gain, illustrated by time period B in FIG. 1. The normal 10-ms transmission frame is illustrated by time period A in FIG. 1. The value of power increment during period B depends on the transmission time reduction method. Thus, as a consequence, the traffic data throughput for the UE is affected negatively when operating in the compressed mode. Since the capacity of a CDMA-network is interference-limited, the increased transmission power in compressed mode reduces the network capacity. Also, the signalling which the compressed mode requires between the UE and the NODE B steals network capacity.